Referring to the block diagram of FIG. 14, an example of a conventional control system for an AC servomotor will be described.
A position deviation is obtained by subtracting a position feedback value detected by an encoder or the like from a position command, and a velocity command is obtained by multiplying the position deviation by a position gain (POSITION CONTROL 1). A velocity deviation is obtained by subtracting a velocity feedback value from the obtained velocity command, and a torque command (current command) is obtained by a process such as proportional-plus-integral control (VELOCITY CONTROL 2). Further, a current deviation is obtained by subtracting a current feedback value from the obtained torque command, and a voltage command for each phase is obtained by a process such as proportional-plus-integral control (CURRENT CONTROL 3). An AC servomotor 4 is controlled by carrying out PWM control or the like in accordance with this voltage command.
Generally, three phase currents are controlled separately for individual phases in a current loop of a control system for a three-phase AC servomotor. Referring now to FIG. 15, therefore, a current loop process for thus separately controlling the three phase currents will be described.
Current commands for the individual phases are obtained by multiplying the torque command (current command) obtained in the VELOCITY CONTROL 2 by sine waves that deviates from a rotor phase .theta. of the servomotor, detected by an encoder or the like, by a margin corresponding to an electrical angle of 2.pi./3 with respect to U-, V- and W-phases. Current deviations are obtained by subtracting actual currents Iu, Iv and Iw for the individual phases, detected by means of a current detector, from the obtained current commands. Proportional-plus-integral (PI) control or the like is carried out by current controllers 5u, 5v and 5w for the individual phases, and command voltages Eu, Ev and Ew for the individual phases are delivered to a power amplifier 6. The power amplifier 6 shown in FIG. 15 carries out PWM control by means of an inverter or the like, and supplies the currents Iu, Iv and Iw for the individual phases to the servomotor 4, thereby driving it. This current control system is called an AC system.
In general, magnetic saturation is caused in a magnetic circuit in an AC servomotor if the current supplied to the motor is increased. Due to this magnetic saturation, the torque constant tends to be lowered despite the substantial current supply, so that it is hard to obtain a desired torque.
Referring now to FIG. 16, the relation between supply current and torque will be described.
If no magnetic saturation is caused in the magnetic circuit in the motor, a torque T that is generated with increase of the supply current increases with a torque constant kt. In the case where magnetic saturation is caused when Iq exceeds the supply current, in contrast with this, the maximum torque (indicated by broken line in the drawing) that can be generated by the motor becomes lower than a torque value to be determined on the basis of the torque constant kt, and the obtained torque is not higher than the maximum torque indicated by broken line in the drawing.
If the magnetic saturation is corrected by the convention AC-system current control, high-speed rotation results in a substantial phase delay even though it is constant-speed rotation. According to the current control of the AC servomotor based on the AC system, it is difficult to carry out control with a correction value for the magnetic saturation separated from a phase delay.
Compared with this AC-system current control, there is a system in which the three phase currents of the motor are converted into two phases, d- and q-phases, by d-q conversion, a d-phase current Id in the direction of a magnetic flux that is generated by a field system is controlled so as to be zero, and the level of only a q-phase current Iq in a direction perpendicular to the current Id is controlled as in the case of a DC servomotor. This system is more improved than the AC system in that it is free from the problem of phase delay because the current is controlled as a direct current. Nevertheless, the problem of the magnetic saturation still remains, and the output torque is lowered under the influence of the magnetic saturation as the current commands are enhanced.